London is huge and there are loads of museums and galleries in there. National Portrait Gallery is one of those smaller ones, compared with, for instance, British Museum and National Gallery (which is just steps away from the Portrait Gallery). So people tend to miss this one. Frankily, I won't say it's a must-see like British Museum or National Gallery. But if you have plenty of time to kill in London then it's probably worth spending couple hours in it. As the name implies, you can only see portraits in this gallery, and some people can easily get bored by this. However, entrance is free so why not pay it a short visit after you've been to the National Gallery??